We analyze output from 900-day runs of the Weather Research and Forecasting Model (WRF). The model is configured as a Cartesian channel with a zonal length of 15,000 km, a meridional width of 10,000, and a 50 km gridscale. The lower boundary is an ocean with imposed, zonally symmetric sea-surface temperatures (SST). Sixteen runs are carried out, in which the tropical SST and its equator-pole temperature contrast are varied independently (Tierney et al., 2021).
The subtropical high in this model is a zonal band of high pressure. Westward propagating waves, that resemble observed waves on the tropical easterly jet, occur on the equatorward side of the subtropical high. These waves extract energy barotropically from the mean flow, and, like observed waves on easterly flows, they sometimes give rise to tropical cyclones (TCs). Model TCs are credible, given the expected limitations imposed by the model resolution.
We examine the sensitivity of westward waves and their generation of TC to climate states. The number of TCs increases, as expected, with overall temperature, but it also varies with baroclinicity, decreasing significantly for the strongest extratropical jets (changing baroclinicity in this model setup does not change tropical and subtropical surface temperatures). Results are analyzed to determine the dynamical mechanisms for these changes, in wave generation focusing on coupling between barotropic influences and vertical shear and stability in the tropics.
Our results demonstrate that westward waves and subsequent TCs can emerge in an atmosphere without regional continental heating or monsoons and their associated localized jets, and allow us to explore the dynamics and climate sensitivity of these waves in the simplest setting that is potentially relevant to Earth’s atmosphere.
- Indicates paper has been withdrawn from meeting
- Indicates an Award Winner